Comparing to a display unit using a CRT (Cathode-Ray tube), an LCD (Liquid Crystal Display) unit is advantageous in that its display screen has been made larger, lighter, and thinner and the power consumption thereof has been reduced. Thus, along with a self-emission type PDP (Plasma Display Panel) or the like, the liquid crystal display unit has frequently been used for a television set or various types of display units. The liquid crystal display unit has a liquid crystal panel which encapsulates liquid crystal between two transparent substrates of various sizes and applies a voltage between electrodes provided on the transparent substrates to change the orientation of liquid crystal molecules to thereby change light transmittance, optically displaying a given image or the like.
Since the liquid crystal itself is not a self-emissive material, the liquid crystal display unit includes a light source section for supplying illumination light to the liquid crystal panel. As the light source section, a sidelight system that irradiates the liquid crystal panel with illumination light from the side position of the panel back surface or a backlight system that directly irradiates the back surface of the liquid crystal panel is typically adopted. The backlight unit has, e.g., a light source, a light guide plate for guiding illumination light to the liquid crystal panel, a reflection sheet, and a lens sheet or diffusion sheet. Using the above optical plates, the backlight unit uniforms the illumination light so as to irradiate the entire surface of the liquid crystal panel with the uniformed illumination light.
Conventionally, as a light source of the backlight unit, a CCLF (Cold Cathode Fluorescent Lamp) which encapsulates mercury, xenon, or the like in a fluorescent tube has been used.
Such a backlight unit has some problems to be solved, with regard to the CCLF. That is, the CCLF cannot provide satisfactory emission brightness; the life of the CCLF is comparatively short; brightness uniformity cannot be secured in the CCLF due to generation of low brightness area on the cathode side thereof; and the like.
A large-sized liquid crystal display unit is typically provided with an area litconfiguration backlight unit having a plurality of long CCLFs disposed on the back side of a diffusion plate for uniforming illumination light emitted from a light source so as to irradiate a liquid crystal panel with the illumination light. Also in this area litconfiguration backlight, solution of the abovementioned problems that the CCLF has is required. Especially, in a large-sized television set of 30 inches or more, problems in terms of brightness level and brightness uniformity have become more prominent.
On the other hand, in the field of the area litconfiguration backlight unit, an LED (Light Emitting Diode) area litconfiguration backlight having, as a light source, a large number of red, green, and blue LEDs arranged on the back surface side of a diffusion film in a two dimensional manner so as to obtain white light has gotten a lot of attention in place of the abovementioned CCLF. In such a LED backlight unit, cost reduction is achieved along with reduced cost of the LED, and high brightness image or the like can be displayed on its large-sized liquid crystal panel with low power consumption.
In a liquid crystal display unit, a large amount of heat is generated from a large number of LEDs provided in the LED backlight unit. An enclosed space of the liquid crystal display unit is formed by close contact between the LED backlight unit and the back surface side of a liquid crystal panel, so that heat from the respective LEDs is accumulated in the enclosed space to produce high-temperature state. As a result, the respective LEDs themselves are heated and thereby their emission characteristics are changed. Accordingly, a change occurs in the color of the light emitted from the LEDs to generate color irregularity or brightness irregularity in the light emitted from the LED backlight unit.
Further, there is a fear that heat generated from the LEDs is accumulated in the enclosed space formed on the back surface side of the liquid crystal panel to deform or transform optical plates. There is also a fear that electronic components or integrated circuit elements are affected by heat to result in unstable operation.
To solve the above disadvantages, the liquid crystal display unit is provided with a heat radiator that effectively radiates heat generated from the respective LEDs provided in the LED backlight unit. A cooling fan is used as this kind of heat radiator. This cooling fan is used to directly blow cooling air over the LEDs of the LED backlight unit to achieve heat radiation. However, the use of such a cooling fan allows dust included in the cooling air sucked from outside the unit to be adhere to the surfaces of the LEDs, thereby deteriorating the emission characteristics of the light emitted from the LEDs.
In order to cope with this problem, a heat radiator configured to perform heat radiation while conducting heat generated from the LEDs from the enclosed space to a heat sink through an appropriate heat conducting member has been proposed. The heat sink is formed by a lightweight metal material having a high heat conductivity, such as aluminum, and is constituted by a plate-like base to be mounted to, e.g., a frame and a large number of fins integrally formed by raising up a part of the surface of the base which faces to the mounting surface thereof and arranged at predetermined intervals from one another. Since the heat conducting member blocks illumination light, such a heat radiator cannot directly be disposed within the enclosed space.
Another type of heat radiator has also been proposed, in which, for example, a wiring substrate on which a large number of LEDs are mounted is mounted to a heat radiation plate which is to be fixed to a back plate, and a heat sink is mounted to the back surface side of the back plate. In this heat radiator, heat from the respective LEDs is conducted to the heat sink through the heat radiation plate and back plate to thereby achieve heat radiation. In addition, a heat radiator having a cooling fan mounted to a heat sink and providing cooling air between respective fins of the heat sink or discharging internal air from between the respective fins to perform effective heat radiation has been proposed.
In order to achieve satisfactory heat conduction between the radiation plate and back plate and between radiation plate and heat sink in a heat radiator, these members need to be coupled to each other in an adhesive manner. The radiation plate or back plate is formed into a flat plate-like shape and thereby can achieve comparatively highly accurate coupling. For example, by combining a high efficiency heat conducting member such as a heat pipe or the like with the radiation plate, more satisfactory heat conduction can be realized.
On the other hand, the heat sink is formed by extruding an aluminum material and is constituted by, as described above, a base serving as a fixing portion to the back plate and a large number of fins integrally formed by raising up a part of one surface of the base. Although the base of the heat sink is comparatively thinly formed in general in order to achieve a reduced thickness and effective conduction of heat to the respective fins, a sufficient mechanical strength can be guaranteed since the respective fins are formed over the entire length of the base. A plurality of fins are integrally formed by the raising processing on the surface of the base, so that the thickness of the heat sink in the width direction is largely changed to generate warpage due to influence of processing accuracy or heat expansion. This makes it difficult to achieve adhesion between the base and back plate throughout the entire surfaces thereof to generate a gap therebetween, thereby deteriorating heat conduction efficiency.
The LED backlight unit is mounted to the inner surface of the back plate so as to face to the liquid crystal panel. The heat sink is mounted to the outside surface of the back plate. In order to reduce weight, thickness, and material cost while attempting to increase processing accuracy and production efficiency, the thickness of the back plate has been reduced in the liquid crystal display unit. This back plate constitutes a mounting member between the heat radiation plate and heat sink together with other various components, as described above.
In order to achieve adhesion between the entire surfaces of the heat radiation plate and heat sink which is comparatively large and heavy and is subject to warpage to guarantee the mechanical strength thereof to thereby increase heat conductivity, fixing members such as screws are used to tightly mount the radiation plate and heat sink. In this case, since each component is slightly warped, deformation occurs in a mounting portion of the back plate whose thickness has been reduced to impair the adhesion between the heat radiation plate and back plate and between the heat radiation plate and heat sink. It follows that heat conduction efficiency is reduced, failing to perform effective heat radiation. Further, provision of the mounting portions to the radiation plate and heat sink on the back plate reduces the mechanical strength of the back plate and increases man-hours to screw up. Further, intervention of the back plate between the heat radiation plate and heat sink impairs mechanical fixation between the heat radiation plate and heat sink as well as impairs heat conductivity therebetween.